The present invention refers to a cutting blade control system, particularly for cutting-off machines for cutting logs of web material.
In the production of rolls of web material, such as paper, tissue paper, non-woven fabric, plastic materials etc., logs or batons are initially produced and must subsequently be cut into rolls of the desired length. For this purpose use is made of special cutting-off machines, sometimes called log saws, which have one or more cutting discs or rotating band saws that are cyclically lowered onto the logs to be cut. The cutting blade is moved towards the logs to be cut, which are conveyed on a conveyor driven by a chain extending around idler wheels.
In disc or band saw cutting-off machines a problem that reduces the production rates, even drastically, is the overheating and the consequent deformation of the cutting blade. Since the cutting blade, to perform the cutting of the log, must have a high speed of rotation during cutting, high friction is generated between the surface of the blade and that of the log to be cut, which causes an excessive overheating of the blade. Once a threshold temperature has been reached, which varies according to the material and to the shape of the cutting disc, a slackening and a thermal dilation of the disc take place, leading to deformation thereof. This deformation is due to the residual tensions induced in the cutting disc during manufacture to cause its surface to take on the desired planarity.
Consequently, once a threshold deformation of the blade has been reached, the cutting of the log must be interrupted to allow the cooling thereof, otherwise the excessively hot blade could be further deformed, becoming damaged or even breaking. Machine idle times to allow the blade to cool are calculated, entirely empirically, on the basis of the operator's professional skill. Consequently, to avoid damages to the blade, there is a tendency to stop the machine within wide safety limits, with the result of a drastic lowering of production.
Cutting-off machines according to the prior art can be programmed, for example, to make a certain number of consecutive cuts and then to stop for a predetermined time to allow the blade to cool. As previously stated, the number of consecutive cuts and the idle time are determined empirically by the operator, according to the type of product to be cut. Furthermore, even operating with the same product, logs of different densities due to irregularities in the grammage of the paper may be cut. Consequently, it is not easy to foresee the moment of the excessive overheating of the blade, which depends upon the density of the web material making up the log.
To overcome these drawbacks, at least in part, the applicants, as described in Italian patent No. 1.318.030, have provided for continuous control of the temperature of the cutting blade by means of temperature sensors.
However, temperature is not a parameter easy to measure with sensors, since the steel disc of the blade is dirty with lubricant, there are often coatings, for example of Teflon, on its surface and the shiny moving surface of the blade does not accurately reflect the signals emitted by some types of temperature sensors. Furthermore, the threshold temperature beyond which no correct operation of the blade is guaranteed varies according to the type of material used to form the blades and to the quality of said blades and thus is difficult to calculate precisely.